Hi Govind,
In order to avoid this error you can check below document:
1635590 - How to make manual journal entries against sales projects?
From the attached screenshot,it is not clear which JEV type and screen variant is being used.
While posting the JEV against projects,please check the Voucher Type and Screen Variant you have entered.
Secondly after making changes as per document, if you do not find any field for Project Task you have to add it via personalization.
Also refer knowledge base article : Manual Entries for Sales Documents
This help document will explain you the usage of the JEV type 00088 and its screen variants.
I hope this helps.
Regards,
Kopal